1. Field of the Invention
The present invention relates to a method for forming a semiconductor film to fabricate semiconductor devices therein, more precisely, it relates to a method for forming an epitaxially grown semiconductor film which is electrically isolated from a supporting substrate.
2. Description of the Related Art
Recently, requests for semiconductor devices which can be used under severe condition of high temperature or high radioactivity are increasing. Such requests are especially strong from industry of space equipments, atomic reactor, engine control and so on. Silicon carbide (SiC) is considered to be a desirable semiconductor material for fabricating devices applied for such use. It is known that silicon loses the property of semiconductor at about 200.degree. C., while silicon carbide maintains its semiconductor property up to more than 600.degree. C. SiC has a superior durability for radioactivity compared to ordinary semiconductor materials.
Silicon carbide, however, is difficult to grow a large single crystal ingot. Therefore, it is heteroepitaxially grown on a substrate such as silicon. The heteroepitaxially grown SiC is necessary to be isolated from the substrate in order to make a semiconductor devices. The isolation is especially important for fabricating a planer type devices, such as field effect transistors (FET).
The isolation of heteroepitaxially grown silicon carbide from a substrate is usually done as follows. FIG. 1 schematically shows a conventional structure of SiC substrate. A non-doped SiC 2 of 3-6 .mu.m thick is grown on a silicon substrate 1, and a SiC layer 3 of desired conductivity type is grown on the non-doped SiC layer 2. The SiC layer 3 is insulated from the silicon substrate 1 by the non-doped SiC layer 2, which has a very high resistivity. This method, however, has some faults. First, it takes a very long time to grow the thick non-doped SiC layer 2. Further, the resistivity of the non-doped SiC decreases when temperature becomes higher than 300.degree. C. So, the important characteristics of silicon carbide device that it can be operated at high temperature is lost. Further detail of such device is given in "LOW-TEMPERATURE HETEROEPITAXY OF .beta.-SiC ON Si (111) SUBSTRATES" by T. Eshita et al, Mat. Res. Soc. Symp/Proc. Vol. 116, 1988, Materials Research Society.
Another method to isolate a SiC layer from its silicon substrate is to use a pn junction of SiC as shown in FIG. 2. An n type SiC layer 4, and a p type SiC layer 5 are subsequently grown over a silicon substrate 1. By the pn junction formed between the two SiC layers 4 and 5, the upper SiC layer 5 is isolated from the substrate 1. But since the isolation effect by pn junction is lost over a temperature of 300.degree. C., this method also can be used for fabricating a device operable in high temperature.
FIG. 3 shows still another method of isolation. In this method, a SiC layer 6 and a silicon dioxide (SiO.sub.2) layer 7 are grown successively over a silicon substrate 1, as shown in FIG. 3(a). A second Si substrate 1' is prepared, and a SiO.sub.2 7' is formed on it by thermal oxidation, as shown in FIG. 3(b). These substrates are stacked facing the SiO.sub.2 layers 7 and 7' to each other, and a pulse voltage is applied between the both substrates 1 and 1', as shown in FIG. 3(c). By this process, the two SiO.sub.2 layers 7 and 7' adhere to each other and become a single SiO.sub.2 layer 7. Then, the upper SiO.sub.2 substrate 1 is removed by etching or polishing as shown in FIG. 3(d).
By the above method of FIG. 3, the SiC layer 6 is isolated by the SiO.sub.2 layer 7. Accordingly, the isolation of the SiC layer from the second substrate 1' is maintained up to very high temperature. But there is still one defect in this method. It will be noticed that, the upper surface 8 of the SiC layer 6 in FIG. 3(d) is the surface which was directly contacted to the removed Si substrate 1 (see FIG. 3(a)). The heteroepitaxially grown SiC crystal, however, includes plenty of crystal imperfections at a portion close to the substrate. Therefore, the portion of the SiC layer 6 close to its upper surface 8 has plenty of crystal imperfections, such as dislocations. Accordingly, it is difficult to obtain a good electrical characteristics of the devices when they are formed in such SiC layer, and yield of the device fabrication decreases.
It is known that the density of crystal imperfections decreases rapidly when the position in the crystal is shifted from the interface between the newly grown crystal and the substrate. For example in a SiC crystal heteroepitaxially grown on a silicon substrate, the density of crystal imperfection is about 1.times.10.sup.10 /cm.sup.2 at a portion 0.2 .mu.m apart from the surface of the substrate, but it reduces to about 10.sup.8 /cm.sup.2 at a portion 1 .mu.m apart from the boundary surface. Accordingly, if it is possible to use a crystal which is grown slightly apart from the surface which was contacted to the substrate when the crystal was grown, the problem of crystal imperfection is largely traversed.